mobile: fully port to QQC2
Summary: Remove all QtQuick Controls 1 imports and replace by QtQuick Controls 2
Test Plan: The mobile app behaves just as before
Reviewers: Okular, ngraham
Reviewed By: ngraham
Subscribers: ngraham, okular-devel
Tags: Okular
Differential Revision: https://phabricator.kde.org/D21675